In a 6-quart slow cooker, place the melted butter, sliced onions, minced garlic, thyme, kosher salt, and black pepper. Cover and cook on the high setting for 5 to 7 hours. Stir occasionally to ensure even caramelization of the onions until they reach a golden, caramelized consistency.

Once the onions are well-cooked, pour in the beef broth and white wine. Cover once again and continue cooking on high for an additional 30 minutes.

While the soup finishes, prepare the bread. Position an oven rack about 6 inches from the broiler, then preheat the broiler. Arrange the French bread slices on a baking sheet, and lightly brush both sides with olive oil. Broil the bread for 2 to 3 minutes per side, closely monitoring and rotating the pan as needed to avoid burning. Remove the toasted slices from the oven.

When the soup is done, ladle it evenly into 4 oven-safe bowls or ramekins. Top each serving with 2 or 3 slices of the toasted bread, and sprinkle about 2 tablespoons of shredded Gruyere cheese over each bowl.

Place the filled bowls on a baking sheet, then broil for about 2 minutes until the cheese is melted, golden, and bubbling. Keep a close watch to prevent over-browning. Sprinkle each serving with a pinch of salt and black pepper, and serve piping hot.